[BUILD] CAPI VP312DI~500/51x Series~Preamp + Direct Inject~Official Support Thread

[eighthsonpro]

That is normal for red dot so no problem there. Just send me an email thru the store and I'll send you a new cap. I would suggest NOT using the DI on post 2622 mode unless the output is cap coupled.

@jsteiger i had a 470uf, 6v3 pop after i powered up one one freshly built 312DI pres. same as the cap in Matt's pic. mine is in C10m though. Can i write you for a couple of replacements?

 

[jsteiger]

@jsteiger i had a 470uf, 6v3 pop after i powered up one one freshly built 312DI pres. same as the cap in Matt's pic. mine is in C10m though. Can i write you for a couple of replacements?

That indicates an opamp problem. I don't currently have stock but the part # is in the BOM. Can hopefully be found at Mouser or Digi.

 

[eighthsonpro]

That indicates an opamp problem. I don't currently have stock but the part # is in the BOM. Can hopefully be found at Mouser or Digi.

thank you for replying!

 

[fritzmyname]

Hello,
I've just finished up a 312DI:
-VPR / 16 V version
- Litz transformer
- CAPI 0252 opamp (kit, not prebuilt) since this is both my first API style pre that uses those kinds of opamps, I couldn't test the opamp and the pre separately from each other
- using the Fredenstein 2 slot rack

Problem: no signal passing thru (tested with SM58 and test signal from Reaper). Only on very high gain settings I can hear a tiny bit of the test signal, but I don't know if this is just the signal passing through unaffected if that makes sense.

There was no smoke or anything else suspicious. I really took my time with this build and I think it is pretty clean. Double checked all the values and everything while building.

opamp socket measurements:
-V & +V: 4 MOhm
- V & C: 8MOhm
+V & C 13 kOhm
C & O: 1 MOhm
+V & O: 500 kOhm
-V & O: 17MOhm

With power / without opamp:
+V - C: 15.5 V DC
-V - C: -16.25 V DC

How would I go about locating the problem? Are there more test points where I should measure voltage or resistance to narrow down the problem area?

Thanks in advance for any help.

I've attached some pics, maybe someone can spot something? I doubt that, because I think everything looks clean.

 

[fritzmyname]

Additional information:

When sending in a test sine wave, it makes a sonic (edit) difference for the output signal if the phantom switch is on or off. (again, on very high gain settings you can hear a little bit on the output). When 48 V is engaged, the output signal is the 400 Hz test tone. When phantom power is off, there is no output except for a static noise that looks like the one in the picture. Also, when phantom is off, I can sometimes hear a tapping when I lightly touch the buttons and knobs. (All tested with a 400 Hz sine wave from Reaper and back).
When I mute the test signal, meaning I only monitor the output without an input signal, but connected to my rig, I get the same static sound / noise / standing waves (see picture) when phantom is disengaged. With phantom on and no input signal, I get no output (as should be).


When I plug in a guitar into the DI, I hear a strong (normal?) signal. The PAD, the Polarity Switch and the Mute work. The Phantom Power Switch makes no difference. This is with the small switches to the left (writing) side. The POST 2622 switch does something to the signal (louder, more harmonics). When the POST 2622 switch is engaged, the -10 dB Pad and lowpass mini switches work.

So...what do we think about that?

I am grateful for every help!

 

[fritzmyname]

Jeff suggested that this problem might be because of issues with one or both relay PCBs. I checked them again and all the diodes seem to be correct (black end is where the arrow points) and the relays are placed, so that the white line is facing upwards / away from the main PCB.

I also hear a clicking sound when turning on the power (can't tell tho if it's two clicks at the same time or just one).

 

[fritzmyname]

Relay testing:
Unit connected to power


PK1 / next to DI jack:
B+ O+ beep when no plug is plugged in
A+ O+ beep when plug is in and the mini toggle is at Post 2622
B+ O+ beep when plug is in
Clicking audible when "post" switch is flipped

PK2 / next to input transformer:
A+ O+ beep when in mic mode
B+ O+ beep when plug is inserted (no matter where "Post 2622" is)
Clicking audible as soon as tip of plug is inserted and touched sleeve contact of the Neutrik

So that's that, seems like the relays all work as they should

 

[fritzmyname]

Ok, so turns out the problem was related to the combination Fredenstein Bento 2 and extender jig... The jig tests fine and the Fredenstein itself too. I'm confused.

Anyways: The unit works, except the phantom power. When plugging in a 57 the signal is strong and sounds amazing. When I turn on phantom power, the signal from the dynamic mic gets thin.
Condenser mics don't work at all.

I traced the +48V from pin 15, to BB8 to BB1, through the switch, to R10. After that, all the resistors measure according to the schematic. I cannot measure phantom power on the XLR input pins.


Could that somehow be the Fredenstein's fault?

 

[thelivingroom]

I just wanna say these sound phenomenal! I’m running mine with the garOA12’s so I can have them on +/-24v in my 51X rack. I also have the garAM10’s that I’ll try in them after they’re assembled. Pre sounds great. DI sounds great. Thank you for the awesome products Jeff!

If you have the Hakko FX-888D iron, the fine point tip with the hook on the end (pictured below) that I bought when redoing a bunch of wiring on my patchbays worked brilliantly on all the small boards, sockets, and op amps.

 

[powelles]

I just built two of the VP312DI and I have an issue with one of them. On the problematic one, the preamp gain seems to be stuck at the lowest setting and unaffected by the stepped gain switch. Everything else seems to be functioning properly. Both are stepped gain versions with pikatron DI's. I can't find any obvious issues like backwards caps, diodes and transformers or obvious solder bridges. I've also tried various opamps to rule that out. I've put a meter on the three pads for the pot version and stepped gain switch and resistor array seem to be working properly. I'm not sure how to go about troubleshooting this further.

 

[powelles]

I think I figured it out. C4, the 120pF/50V cap is not working properly. It reads just like it was a piece of wire.
Edit: Confirmed. Preamp worked perfectly after replacing C4 with a new cap. I've heard caps like that being bad is extremely rare. So this isn't likely to be anybody else's fix.

 

[Clbraddock]

What size are the countersink screws and black plastic washers for the L bracket? One of mine fell into the abyss to never be seen again.

Edit: figured out a solution

 

[Clbraddock]

Is there any info about the pad impedance and modification? On the vp312 product page it says:

"Notes on the schematic give the builder a choice of attenuation levels, with the standard and original version being -20db. You can choose from a few options calculated to represent similar source and load impedances to the ones that exist in the circuit before the pad is engaged."

I don't see that info in the schematic, but maybe I am looking in the wrong place. If I want to use the pad primarily for line level signals is the stock -20db configuration the best option or would a modification be better?

 

[jsteiger]

Is there any info about the pad impedance and modification? On the vp312 product page it says:

"Notes on the schematic give the builder a choice of attenuation levels, with the standard and original version being -20db. You can choose from a few options calculated to represent similar source and load impedances to the ones that exist in the circuit before the pad is engaged."

I don't see that info in the schematic, but maybe I am looking in the wrong place. If I want to use the pad primarily for line level signals is the stock -20db configuration the best option or would a modification be better?

I'd go with two 4k99s and a 160R shunt. Basically replace the 750Rs with 4k99s. That will be about -40dB.

 

[Clbraddock]

I'd go with two 4k99s and a 160R shunt. Basically replace the 750Rs with 4k99s. That will be about -40dB.

Thank you for the reply jsteiger! Would that mod change the impedance or just increase the pad amount? All of the line signals in my setup are coming from my computer interface, so I can control the level going to the hardware insert from there. I mostly want to make sure the impedance is okay for line level signals. (i.e. if the impedance stock is okay for line signals Im fine leaving it stock. If modding it would be better impedance-wise I'll do your suggested swap) Thanks!

 

[jsteiger]

Thank you for the reply jsteiger! Would that mod change the impedance or just increase the pad amount? All of the line signals in my setup are coming from my computer interface, so I can control the level going to the hardware insert from there. I mostly want to make sure the impedance is okay for line level signals. (i.e. if the impedance stock is okay for line signals Im fine leaving it stock. If modding it would be better impedance-wise I'll do your suggested swap) Thanks!

That will make the input Z just under 10k.

 

[PaulSmith]

I’m having some issues with my recent build - hoping Jeff can help me out!

- DI mode worked but the first few steps of Grayhill switch didn’t change gain.

- Resoldered grayhill switch/resistors then it worked fine for a bit but when I tested mic input all I got is lots of microphonic noise when changing gain/switches and no output. Now no output from either DI or Mic…. Checked op-amp and that’s good.

Any help greatly appreciated!

 

[jsteiger]

I’m having some issues with my recent build - hoping Jeff can help me out!

- DI mode worked but the first few steps of Grayhill switch didn’t change gain.

- Resoldered grayhill switch/resistors then it worked fine for a bit but when I tested mic input all I got is lots of microphonic noise when changing gain/switches and no output. Now no output from either DI or Mic…. Checked op-amp and that’s good.

Any help greatly appreciated!

Considering the Grayhill switch fix I'd assume more cold/bad solder joints elsewhere on the build.

 

[PaulSmith]

Thanks for this. I did check and all appear pretty good - I reflowed basically every joint. It appears as though I was wrong though and that the problem is something to do with the input transformer, as the DI works great when I engage the post-2622 mini toggle?

Still no sound in mic mode however.

Many thanks for your help!

 

[jsteiger]

Thanks for this. I did check and all appear pretty good - I reflowed basically every joint. It appears as though I was wrong though and that the problem is something to do with the input transformer, as the DI works great when I engage the post-2622 mini toggle?

Still no sound in mic mode however.

Many thanks for your help!

In the past folks have had problems with the 2 relay boards not switching. Wrong facing diodes or getting the 4V3 zener in the wrong spot.